1. Field of the Invention
The present invention relates to a silicone-oil soluble polymer and to an image display medium using the silicone-oil soluble polymer. More particularly the present invention relates to an image display medium in which an image is reversibly displayed by the movement of charged white or colored particles upon application of an electric field thereto. In addition the present invention relates to an image display device.
2. Discussion of the Background
Display devices such as cathode ray tubes (CRTs) and liquid crystal displays (LCDs) have been used as an image display terminal for displaying images such as letters, static images and moving images. These devices can instantaneously display images (i.e., digital data) and rewrite the images. However, these devices have various drawbacks. For, example, it is a hard work to always carry such devices. In addition, to see such a display device for a long period of time causes eye fatigue. Further, when such a display device displaying images is turned off, the images are disappeared.
When images such as letters and static images are distributed or stored as a document, the images are typically recorded by a printer on a paper medium to form a hardcopy. Images on such a hardcopy are easier to read than those in the display devices mentioned above. In addition, when reading images on hardcopies, the degree of eye fatigue is much less than that when reading images in the display devices. Further, a reader can see images on hardcopies while having a free posture. Furthermore, hardcopies can be easily carried because of having lightweight. However, hardcopies have a problem in view of material saving such that much labor and costs are necessary to recycle used hardcopies.
Therefore, a need has been existed for a paper-like image display medium having a combination of the advantages of the above-mentioned display devices and the advantages of hardcopies. Until now, display media using a liquid crystal dispersed in a polymer, a bistable cholesteric liquid crystal, an electrochromic material or an electrophoretic material have been proposed. These media are a reflection type and therefore can display high contrast images. In addition, the media can be used as a memorable display medium (i.e., the media have an image maintaining ability even when the power is off). Among these media, devices using an electrophoretic material have good display qualities and low power consumption and therefore are advantageous over other media. A typical embodiment of the electrophoretic display media, which is disclosed in Japanese Laid-Open Patent Publication No. 5-173194 or Japanese Patent No. 2,612,472 (i.e., Japanese Laid-Open Patent Publication No. 1-300231), is illustrated in FIG. 3.
In FIG. 3, numerals 6 and 6′ denote transparent substrates made of glass. Numerals 7 and 7′ denotes pattern electrodes formed on the substrates 6 and 6′, respectively. A dispersion 8 including a dispersing medium and electrophoretic particles whose color is different from the color of the dispersing medium are contained in the cell formed of the pair of substrates 6 and 6′. The surfaces of the electrophoretic particles are charged in the dispersing medium. When a voltage having a polarity opposite to the charge of the electrophoretic particles is applied to the electrode 7, the particles move toward the electrode 7, resulting in deposition of the particles on the electrode 7. To the contrary, when a voltage having the same polarity as the charge of the electrophoretic particles is applied to the electrode 7, the particles move toward the substrate 6′, and thereby the color of the dispersing medium can be seen from the side of the electrode 7. Thus, images can be reversibly displayed. When the dispersion 8 is simply contained in the cell without providing spacers 9, uneven images are often displayed due to aggregation of the electrophoretic particles and adhesion of the particles to the electrode 7. Therefore, a mesh-like or porous spacer 9 is preferably provided in the cell to stably display images. In the display device having such a construction, it is difficult to evenly contain the dispersion into the thus separated cells. Alternatively, there occurs a problem such that when the dispersion is contained in the cell, the properties of the dispersion often change, and thereby it becomes difficult to stably produce a display medium capable of displaying images having a desired image qualities.
It is well known that the dispersion of the particles can be stabilized by electrostatic repulsion effect and a stereo effect (i.e., a so-called adsorption layer effect). With respect to the electrostatic repulsion effect, a DLVO theory is established. In this theory, the thickness of the electric double layer and the phase boundary potential (i.e., the so-called ζ potential) are important factors. Therefore, ions for forming such an electric double layer and a phase boundary potential are needed. Several investigations have been made for aqueous dispersion in which ions can be present.
With respect to the stereo effect, a theory corresponding to the DLVO theory has not yet been established. An investigation using a non-aqueous dispersion system (e.g., petroleum solvent dispersion systems) is reported by F. A. Waite in J. Oil Col. Chem. Assoc., 54, 342 (1971). This investigation relates to a fundamental method for manufacturing a stable non-aqueous solvent dispersion. In this method, a block or graft copolymer which has both a component having an affinity for the particles dispersed in a dispersing medium and another component soluble in the dispersing medium is used.
Japanese Patent Publication No. (hereinafter referred to as JPP) 40-7047 discloses a method for manufacturing a dispersion, which utilizes the above-mentioned method. In the method, methyl methacrylate is radically polymerized in a hydrocarbon solvent in the presence of a degradable rubber to prepare a stable polymethyl methacrylate dispersion. It is considered that methyl methacrylate is graft-copolymerized with the degradable rubber because it is unlikely that the polymethyl methacrylate particles adsorb the degradable rubber. In addition it is considered that when the graft copolymer is included in a display dispersion, the insoluble part of the graft copolymer associates with the surface of particles therein, and the soluble part of the graft copolymer has a stereo effect. Thus, the dispersion stability of the particles can be maintained.
However, a method by which solid particles are stable dispersed in a non-aqueous solvent such as nonpolar aprotic solvent (e.g., petroleum solvents) by ionically charging the solid particles is not yet known. Therefore, dispersions used for electrodeposition paints, electrophotographic liquid developers and display devices have a short life.
In attempting to solve this problem, JPP 8-23005 (i.e., Japanese Laid-Open Patent Publication No. 59-213428) discloses a dispersion in which solid particles are charged in a solvent, such as aliphatic hydrocarbons and aromatic hydrocarbons, using an acid-base ion dissociation. On the other hand, it is preferable not to use solvents such as hydrocarbons and aromatic hydrocarbons in view of environmental protection. Therefore, now silicone oils are used as a substitute of such solvents. However, a method by which solid particles are ionically charged in a silicone oil to stably disperse the particles therein is not yet known until now. Therefore, if the technique of the dispersion described in JPP 8-23005 is merely applied to a dispersion including a silicone oil, a stable dispersion cannot be obtained. Namely, a material which can effectively charge solid particles in a silicone oil is not yet known.
Because of these reasons, a need exists for an image display medium which can reversibly display high quality images for a long period of time even when a silicone oil is used as a dispersing medium and which has an image maintaining ability and high response.